This invention relates to a fuel injector for use in supplying fuel, under pressure, to a combustion space of a compression ignition internal combustion engine.
In order to reduce emissions levels, it is known to provide fuel injectors in which the total area of the openings through which fuel is delivered can be varied, in use. One technique for achieving this is to use two valve needles, one of which is slidable within a bore provided in the other of the needles to control the supply of fuel to some of the outlet openings independently of the supply of fuel to others of the outlet openings.
A known fuel injector of this type includes an outer valve needle which is provided with a through bore within which an inner valve needle is slidable, the outer valve needle being slidable within a bore provided in a fuel injector nozzle body. The nozzle body is provided with first and second outlet openings, occupying different axial positions in the nozzle body. A valve insert member is received within the through bore provided in the outer valve needle, the lower end surface of the valve insert member, the bore provided in the outer valve needle and an upper surface of the inner valve needle together defining a spring chamber which houses a compression spring, the spring serving to urge the inner valve needle against the second seating.
When the outer valve needle is moved away from the first seating by an amount less than a predetermined amount, fuel is delivered through the
first outlet opening and the inner valve needle remains seated to prevent fuel delivery through the second outlet opening. When the outer valve needle is moved away from the first seating by an amount greater than the predetermined amount, a surface of the outer valve needle engages an enlarged region of the inner valve needle, movement of the outer valve needle thereby being transmitted to the inner valve needle causing the inner valve needle to move away from the second seating to permit fuel delivery through the second outlet opening. In this way, the fuel delivery rate, or other injection characteristic, can be varied, in use, by controlling the extent of movement of the outer valve needle away from its seating.
Fuel injectors of this type do, however, suffer from the disadvantage that, during the non-injecting stages of the injection cycle, fuel may be able to escape from the spring chamber, thereby causing poor emissions. Additionally, exhaust gases from the engine cylinder may be able to enter the spring chamber which can degrade the performance of the fuel injector. The inner valve needle is also subjected to undesirably high stresses during operation, particularly just prior to the inner valve needle being moved away from the second seating to expose the second outlet opening.
It is an object of the present invention to provide a fuel injector which alleviates one or more of the aforementioned problems.
According to a first aspect of the present invention there is provided a fuel injector comprising a nozzle body having a first bore defining first and second seatings, an outer valve needle being slidable within the first bore and engageable with the first seating to control fuel flow from a first outlet opening, the outer valve needle being provided with a second bore within which an inner valve needle is slidable, the inner valve needle being engageable with the second seating to control fuel delivery through a second outlet opening, the outer valve needle including a deformable region which is shaped such that, in use, when the outer valve needle is urged against the first seating, the outer valve needle deforms.
In one embodiment of the invention, the deformable region is shaped such that, in use, when the outer valve needle is urged against the first seating, the outer valve needle cooperates with the inner valve needle to form a substantially fluid tight seal.
By providing the outer valve needle with the deformable region, when the outer valve needle is seated against the first seating the volume defined by the inner valve needle, the outer valve needle and the fuel injector nozzle body within which fuel can reside is significantly reduced. Thus, a reduced volume of fuel is exposed to exhaust gases from the engine cylinder or other combustion space, thereby improving the performance of the fuel injector.
Alternatively, or in addition, the deformable region may be shaped such that, in use, when the outer valve needle is urged against the first seating, the outer valve needle deforms to close the first outlet opening.
Conveniently, a chamber is defined within the second bore, cooperation between the deformable region of the outer valve needle and the inner valve needle when the outer valve needle is urged against the first seating causing the chamber to be substantially sealed.
As the chamber is sealed when the outer valve needle is seated against the first seating, exhaust gases from the engine cylinder or other combustion space are prevented from entering the chamber. This improves the performance of the fuel injector. Additionally, as fuel is unable to escape from the fuel injector when the outer valve needle is seated against the first seating, leakage of fuel from the fuel injector during undesirable stages of the fuel injecting cycle is substantially avoided.
Conveniently, the inner valve needle and the outer valve needle may be arranged such that movement of the outer valve needle away from the first seating beyond a predetermined amount is transmitted to the inner valve needle, thereby causing the inner valve needle to move away from the second seating.
The outer valve needle may be provided with a surface which is engageable with a first region of the inner valve needle to transmit movement of the outer valve needle to the inner valve needle. The first region and the surface are preferably of substantially frusto-conical form.
Preferably, the surface of the outer valve needle which is engageable with the first region is located on the outer valve needle at a position remote from the deformable region. As the surface is located remotely from the deformable region, towards the uppermost open end of the second bore, the fuel injector is easier to manufacture.
The inner valve needle may further comprise a second region located downstream of the first region, the second region being of substantially frusto-conical form such that stresses within the second region of the inner valve needle are minimized upon engagement between the surface of the outer valve needle and the first region of the inner valve needle.
The inner valve needle may be slidable within a lower region of the second bore and a valve insert member may be received within an upper region of the second bore, the valve insert member being engageable with a seating defined by the open end of the second bore remote from the inner valve needle to permit fuel upstream of the inner valve needle to vent from the second bore.
According to a second aspect of the present invention there is provided a fuel injector comprising a nozzle body having a first bore defining first and second seatings, an outer valve needle slidable within the first bore and engageable with the first seating to control fuel flow from a first outlet opening, the outer valve needle being provided with a second bore within which an inner valve needle is slidable, the inner valve needle being engageable with the second seating to control fuel delivery through a second outlet opening, the inner valve needle comprising a first region which is engageable with a surface defined by the second bore such that movement of the outer valve needle away from the first seating beyond a predetermined amount is transmitted to the inner valve needle when the surface engages the first region, and comprising a second region located downstream of the first region, the second region being of substantially frusto-conical form such that stresses within the second region of the inner valve needle are minimized upon engagement between the surface of the outer valve needle and the first region of the inner valve needle. By providing the inner valve needle with the second region of substantially frusto-conical form, stresses which are transmitted to the inner valve needle just prior to movement of the outer valve needle can be reduced.
According to a third aspect of the present invention there is provided a fuel injector comprising a nozzle body having a first bore defining first and second seatings, an outer valve needle slidable within the first bore and engageable with the first seating to control fuel flow from a first outlet opening, the outer valve needle being provided with a second bore, an inner valve needle being slidable within a lower region of the second bore and a valve insert member being received within an upper region of the second bore, the valve insert member being engageable with a seating defined by the open end of the second bore remote from the inner valve needle to permit fuel upstream of the inner valve needle to vent from the second bore.
As the seating with which the valve insert member is engageable is located at the open end of the second bore, the seating is easier to manufacture.
The fuel injector may further comprise a spacer member, received within the second bore provided in the outer valve needle, the spacer member being interposed between the inner valve needle and the valve insert member. The spacer member and the valve insert member may be integrally or separately formed.